OpenPGP Working Group                                          M. Elkins
draft-ietf-openpgp-mime-01.txt                  Network Associates, Inc.
Obsoletes: 2015                                             D. Del Torto
                                                 CryptoRights Foundation
                                                               R. Levien
                                    University of California at Berkeley
                                                             T. Roessler
                                                              April 2000

                       MIME Security with OpenPGP

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

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   Copyright (C) The Internet Society 2000.  All Rights Reserved.


   This document describes how the OpenPGP Message Format [1] can be
   used to provide privacy and authentication using the Multipurpose
   Internet Mail Extensions (MIME) security content types described in
   RFC1847 [2].

   This draft is being discussed on the "ietf-openpgp" mailing list.  To
   join the list, send a message to <> with
   the single word "subscribe" in the subject.  A web site containing an
   archive of the list can be found at <

1.  Introduction

   Work on integrating PGP (Pretty Good Privacy) with MIME [3]
   (including the since withdrawn application/pgp content type) prior to
   RFC 2015 suffered from a number of problems, the most significant of
   which is the inability to recover signed message bodies without
   parsing data structures specific to PGP.  RFC 2015 makes use of the
   elegant solution proposed in RFC1847, which defines security
   multipart formats for MIME. The security multiparts clearly separate
   the signed message body from the signature, and have a number of
   other desirable properties. This document revises RFC 2015 to adopt
   the integration of PGP and MIME to the needs which emerged during the
   work on the OpenPGP specification.

   This document defines three content types for implementing security
   and privacy with OpenPGP: application/pgp-encrypted, application/pgp-
   signature and application/pgp-keys.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in
    RFC 2119.

2.  OpenPGP data formats

   OpenPGP implementations can generate either ASCII armor (described in
   [1]) or 8-bit binary output when encrypting data, generating a
   digital signature, or extracting public key data.  The ASCII armor
   output is the REQUIRED method for data transfer.  This allows those
   users who do not have the means to interpret the formats described in
   this document to be able to extract and use the OpenPGP information
   in the message.

   When the amount of data to be transmitted requires that it be sent in
   many parts, the MIME message/partial mechanism SHOULD be used rather
   than the multipart ASCII armor OpenPGP format.

3.  Content-Transfer-Encoding restrictions

   Multipart/signed and multipart/encrypted are to be treated by agents
   as opaque, meaning that the data is not to be altered in any way [2],
   [7]. However, many existing mail gateways will detect if the next hop
   does not support MIME or 8-bit data and perform conversion to either
   Quoted-Printable or Base64.  This presents serious problems for
   multipart/signed, in particular, where the signature is invalidated
   when such an operation occurs.  For this reason all data signed
   according to this protocol MUST be constrained to 7 bits (8- bit (8-bit data
   MUST be encoded using either Quoted-Printable or Base64).  Note that
   this also includes the case where a signed object is also encrypted
   (see section 6).  This restriction will increase the likelihood that
   the signature will be valid upon receipt.

   Data that is ONLY to be encrypted is allowed to contain 8-bit
   characters and therefore need not be converted to a 7-bit format.

      Implementor's note: It cannot be stressed enough that applications
      using this standard follow MIME's suggestion that you "be
      conservative in what you generate, and liberal in what you
      accept."  In this particular case it means it would be wise for an
      implementation to accept messages with any content-transfer-
      encoding, but restrict generation to the 7-bit format required by
      this memo.  This will allow future compatibility in the event the
      Internet SMTP framework becomes 8-bit friendly.

4.  OpenPGP encrypted data

   Before OpenPGP encryption, the data is written in MIME canonical
   format (body and headers).

   OpenPGP encrypted data is denoted by the "multipart/encrypted"
   content type, described in [1], [2], and MUST have a "protocol" parameter
   value of "application/pgp-encrypted".  Note that the value of the
   parameter MUST be enclosed in quotes.

   The multipart/encrypted MUST consist of exactly two parts. The first
   MIME body part must have a content type of "application/pgp-
   encrypted".  This body contains the control information.  A message
   complying with this standard MUST contain a "Version: 1" field in
   this body. Since the OpenPGP packet format contains all other
   information necessary for decrypting, no other information is
   required here.

   The second MIME body part MUST contain the actual encrypted data.  It
   must be labeled with a content type of "application/octet-stream".

   Example message:

        From: Michael Elkins <>
        To: Michael Elkins <>
        Mime-Version: 1.0
        Content-Type: multipart/encrypted; boundary=foo;

        Content-Type: application/pgp-encrypted

        Version: 1

        Content-Type: application/octet-stream

        -----BEGIN PGP MESSAGE-----
        Version: 2.6.2

        -----END PGP MESSAGE-----


5.  OpenPGP signed data

   OpenPGP signed messages are denoted by the "multipart/signed" content
   type, described in [2], with a "protocol" parameter which MUST have a
   value of "application/pgp-signature" (MUST be quoted) if the message
   contains a single signature, or "multipart/mixed" if the message
   contains two or more signatures [8].  In the latter case, each
   OpenPGP signature is denoted using the content-type "application/pgp-
   signature" inside the multipart/mixed.


   The "micalg" parameter for the "application/pgp-signature" protocol
   MUST contain exactly one hash-symbol.
          This hash-symbol of the format "pgp-<hash-
   symbol>", where <hash-symbol> identifies the message integrity check Message Integrity Check
   (MIC) algorithm used to generate the subsequent signature.  Hash-
          symbols  Hash-symbols are
   constructed from the text names registered in [4] [1] or according to the
   mechanism defined in that document by converting the text name to
   lower case and prefixing it with the four characters "pgp-".


   Currently defined values are "pgp-md5", "pgp-sha1", "pgp- ripemd160",
   "pgp-md2", "pgp-tiger192", and "pgp-haval-5-160".

var2      The "micalg" parameter for the "application/pgp-signature"
          protocol MUST have a value of "pgp-<hash-symbol>", where
          <hash-symbol> identifies the Message Integrity Check (MIC)
          algorithm used to generate the signature. The currently-
          defined values for <hash-symbol> are "md5" for the MD5
          checksum, and "sha1" for the SHA-1 algorithm (FIPS 180-1) [5]:
          compliant implementations MUST be able to accept and generate
          MD5 signature-hashes and MUST be able to accept and generate
          SHA-1 signature-hashes.

var2      Note: new <hash-symbol> values may be registered by sending
          email to <>.  The listing of
          current values may be obtained by sending email to <pgp-hash-

   The multipart/signed body MUST consist of exactly two parts.  The
   first part contains the signed data in MIME canonical format,
   including a set of appropriate content headers describing the data.

   The second body MUST contain the OpenPGP digital signature(s). It
   MUST be labeled with a content type of "application/pgp-signature" if
   there is a single signature, or "multipart/mixed" if there are two or
   more signatures.

   When the OpenPGP digital signature is generated:

   (1)  The data to be signed must first be converted to its content-
        type specific canonical form.  For text/plain, this means
        conversion to an appropriate character set and conversion of
        line endings to the canonical <CR><LF> sequence.

   (2)  An appropriate Content-Transfer-Encoding is then applied.  In
        particular, if any line begins with the string "From", it is
        strongly recommended that Quoted-Printable encoding be applied
        and that at least one of the characters in the string is encoded
        using the hexadecimal coding rule.  This is because many mail
        transfer agents treat "From " (the word "from" followed
        immediately by a space character) as the start of a new message
        and thus insert a right angle-bracket (>) in front of any line
        beginning with "From" to distinguish this case, invalidating the
        signature.  In addition, line endings in the encoded data MUST
        use the canonical <CR><LF> sequence where appropriate (note that
        the canonical line ending may or may not be present on the last
        line of encoded data and MUST NOT be included in the signature
        if absent).

   (3)  MIME content headers are then added to the body, each ending
        with the canonical <CR><LF> sequence.

   (4)  As described in [1], [2], the digital signature MUST be calculated
        over both the data to be signed and its set of content headers.

   (5)  The signature MUST be generated detached from the signed data so
        that the process does not alter the signed data in any way.

      Note: The accepted OpenPGP convention is for signed data to end
      with a <CR><LF> sequence.  Note that the <CR><LF> sequence
      immediately preceding a MIME boundary delimiter line is considered
      to be part of the delimiter in [3], 5.1.  Thus, it is not part of
      the signed data preceding the delimiter line.  An implementation
      which elects to adhere to OpenPGP convention has to make sure it
      inserts a <CR><LF> pair on the last line of the data to be signed
      and transmitted (signed message and transmitted message MUST be

   Example message:

        From: Michael Elkins <>
        To: Michael Elkins <>
        Mime-Version: 1.0
        Content-Type: multipart/signed; boundary=bar; micalg=pgp-md5;

      & Content-Type: text/plain; charset=iso-8859-1
      & Content-Transfer-Encoding: quoted-printable
      & =A1Hola!
      & Did you know that talking to yourself is a sign of senility?
      & It's generally a good idea to encode lines that begin with
      & From=20because some mail transport agents will insert a greater-
      & than (>) sign, thus invalidating the signature.
      & Also, in some cases it might be desirable to encode any   =20
      & trailing whitespace that occurs on lines in order to ensure  =20
      & that the message signature is not invalidated when passing =20
      & a gateway that modifies such whitespace (like BITNET). =20
      & me


        Content-Type: application/pgp-signature

        -----BEGIN PGP MESSAGE-----
        Version: 2.6.2

        -----END PGP MESSAGE-----

   The "&"s in the previous example indicate the portion of the data
   over which the signature was calculated.

   Upon receipt of a signed message, an application MUST:

   (1)  Convert line endings to the canonical <CR><LF> sequence before
        the signature can be verified.  This is necessary since the
        local MTA may have converted to a local end of line convention.

   (2)  Pass both the signed data and its associated content headers
        along with the OpenPGP signature to the signature verification

6.  Encrypted and Signed Data

   Sometimes it is desirable to both digitally sign and then encrypt a
   message to be sent.  This protocol allows for two methods of
   accomplishing this task.

6.1.  RFC1847 Encapsulation

   In [2], it is stated that the data is first signed as a
   multipart/signature body, and then encrypted to form the final
   multipart/encrypted body.  This is most useful for standard MIME-
   compliant message forwarding.


       Content-Type: multipart/encrypted;
          protocol="application/pgp-encrypted"; boundary=foo

       Content-Type: application/pgp-encrypted

       Version: 1

       Content-Type: application/octet-stream

       -----BEGIN PGP MESSAGE-----
     & Content-Type: multipart/signed; micalg=pgp-md5
     &     protocol="application/pgp-signature"; boundary=bar
     & --bar
     & Content-Type: text/plain; charset=us-ascii
     & This message was first signed, and then encrypted.
     & --bar
     & Content-Type: application/pgp-signature
     & -----BEGIN PGP MESSAGE-----
     & Version: 2.6.2
     & jJV5bNvkZIGPIcEmI5iFd9boEgvpirHtIREEqLQRkYNoBActFBZmh9GC3C041WGq
     & uMbrbxc+nIs1TIKlA08rVi9ig/2Yh7LFrK5Ein57U/W72vgSxLhe/zhdfolT9Brn
     & HOxEa44b+EI=
     & =ndaj
     & -----END PGP MESSAGE-----
     & --bar--
       -----END PGP MESSAGE-----


   (The text preceded by '&' indicates that it is really encrypted, but
   presented as text for clarity.)

6.2.  Combined method

   The OpenPGP packet format [1] describes a method for signing and
   encrypting data in a single OpenPGP message.  This method is allowed
   in order to reduce processing overhead and increase compatibility
   with non-MIME implementations of OpenPGP. The resulting data is
   formatted as a "multipart/encrypted" object as described in Section

   Messages which are encrypted and signed in this combined fashion are
   REQUIRED to follow the same canonicalization rules as for
   multipart/signed objects.

   It is explicitly allowed for an agent to decrypt a combined message
   and rewrite it as a multipart/signed object using the signature data
   embedded in the encrypted version.

7.  Distribution of OpenPGP public keys

   Content-Type: application/pgp-keys
   Required parameters: none
   Optional parameters: none

   This is the content type which SHOULD be used for relaying public key

8.  Security Considerations

   Use of the protocols defined in this document has the same security
   considerations as OpenPGP, and is not known to either increase or
   decrease the security of messages using it; see [3], [4] for more

9.  Notes

   "PGP" and "Pretty Good Privacy" are registered trademarks of Network
   Associates, Inc.

10.  Acknowledgements

   This draft document relies on the work of the IETF's OpenPGP Working
   Group's definitions of the OP Message Format. The OP message format
   is currently described in RFC 2440 [4]. [1].

   Special thanks are due: to Philip Zimmermann for his original and
   ongoing work on PGP; to Charles Breed for originally proposing the
   formation of the OpenPGP Working Group; and to Steve Schoenfeld for
   helpful feedback during the draft process. The authors would also
   like to thank the engineers at Pretty Good Privacy, Inc (now Network
   Associates, Inc), including Colin Plumb, Hal Finney, Jon Callas, Mark
   Elrod, Mark Weaver and Lloyd Chambers, for their technical

   Additional thanks are due to Jeff Schiller and Derek Atkins for their
   continuing support of strong cryptography and PGP freeware at MIT; to
   Rodney Thayer of Sable Technology; to John Noerenberg, Steve Dorner
   and Laurence Lundblade of the Eudora team at QUALCOMM, Inc; John
   Gilmore, Hugh Daniel and Fred Ringel (at Rivertown) for their timely
   critical commentary; and to the international members of the IETF's
   OpenPGP mailing list, including William Geiger, Lutz Donnerhacke and
   Kazu Yamamoto. The idea to use multipart/mixed with multipart/signed
   has been attributed to James Galvin. Finally, our gratitude is due to
   the many members of the "Cypherpunks," "Coderpunks" and "PGP-USERS"
   mailing lists and the many users of PGP worldwide for helping keep
   the path to privacy open.

11.  Addresses of the Authors and OpenPGP Working Group Chair

   The OpenPGP working group can be contacted via the current chair:

   John W. Noerenberg II
   Qualcomm, Inc.
   5775 Morehouse Dr.
   San Diego CA   92121     USA
   Tel:   +1 619 658 3510

   The principal authors of this draft are:

   Michael Elkins
   Network Associates, Inc.
   3415 S. Sepulveda Blvd Suite 700
   Los Angeles     CA   90034  USA
   Tel:   +1.310.737.1623
   Fax:   +1.310.737.1755

   Raph Levien
   University of California at Berkeley
   579 Soda Hall
   Berkeley  CA   94720  USA
   Tel:   +1.510.642.6509

   Dave Del Torto
   CryptoRights Foundation
   80 Alviso Street, Mailstop: CRF
   San Francisco  CA   94127     USA
   Tel:   +1.415.334.5533, vm: #2
   Thomas Roessler
   Nordstrasse 99
   D-53111 Bonn
   Tel: +49-228-638007


   [1]  Callas, J., Donnerhacke, L., Finney, H., Thayer, R., "OpenPGP
        Message Format", RFC 2440, November 1998.

   [2]  Galvin, J., Murphy, G., Crocker, S., and N. Freed, "Security
        Multiparts for MIME: Multipart/Signed and Multipart/Encrypted",
        RFC 1847, October 1995.

   [3]  Freed, N., Borenstein, N., "Multipurpose Internet Mail
        Extensions (MIME) Part Two: Media Types", RFC 2046, November

   [4]  Galvin, J., Murphy, G., Crocker, S., and N. Freed, "MIME Object
        Security Services", RFC 1848, October 1995.

   [5]  Atkins, D., Stallings, W., and P. Zimmermann, "PGP Message
        Exchange Formats", RFC 1991, August 1996.

   [6]  Elkins, M., "MIME Security with Pretty Good Privacy (PGP)", RFC
        2015, October 1996.

   [7]  Freed, N., "Gateways and MIME Security Multiparts", RFC 2480,
        January 1999.

   [8]  Roessler, T., Del Torto, D., Levien, R., "Multiple Signatures
        using Security Multiparts", draft-ietf-multsig-00.txt, January

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